This article is timely as I wasabout to tackle the same set of issues. Thus I will use this as a back drop to my own ruminations.
First, we have shown that the iceage itself plausibly ended as a consequence of the crust been shifted thirtydegrees south with the original North Pole ending up in Hudson Bay . This conjecture iswritten up extensively here under the heading Pleistocene Nonconformity.
Thus we have a powerful argumentfor almost all of the mega fauna been destroyed then and there with survivorssuccumbing to the environmental disruption that arises with the removal of themain players.
The removal of the buffaloassisted the restructuring of the Great Plains with a sharp change in active participants that today we are now learning toremedy.
Without the herds of mammoths,the forests regrew and choked out open grasslands that would have supported alarge suite of herbivores.
The problem I have with that isthat remnant populations will survive and certainly did. The elimination of those populations mostlikely was due to human predation, or that is the only reasonable explanation availableto us. Except we then ask the questionof why the African and Indian elephants survive. Certainly forest cover made the mammoth morevulnerable to human hunting by establishing game trails that could be made overinto traps.
Quite simply, the elephant is neithereasy to kill and it produces so much meat that a large group is necessary toexploit it. Yet it was done occasionallyin Africa .
I have to conclude that thepopulation was both decimated and that humanity bounced back fast enough to overwhelmthe remaining population.
That still leaves us withpopulations in South America that I assume were strong enough to hold this offif the extinction event were merely local to the northern Hemisphere. A crustal shift is a global event able toimpact global populations and in the nonce, Africa was the least effected been on the axis of rotation. That is not true for the Indian Elephant, buttropical conditions may have served to protect survivors.
Survivors from the remainder ofthe menageries likely died off as the loss of the mammoth hugely altered theenvironment.
Let us understand what that allimplies. The Boreal forest is solidwoodland. Mammoths would tear down mostof these woodlands to produce open country with grasslands leaving country attractivefor a full range of herbivores, rather that the present subdued populations.
The impact conjecture is stillbeen kicked back and forth in academe and no one has yet connected the dots tounderstand just how deliberate it all was. Yet it is able to explain the collapse and the ending of the Ice Ageitself. Without the shock removal of themega fauna, the Boreal forest would be well populated with them and plausiblyuninhabitable by humanity.
Mammoth mystery: Why giants no longer rule the north
31 March 2011 by HenryNicholls
Magazine issue 2805.
Woolly behemoths ruled the frozen steppe for hundreds of thousands ofyears. Were they wiped out by climate change, a killer asteroid – or ourancestors?
IN 1643, workers unearthed some huge bones in a field outside Bruges in Belgium
In 1728, British anatomist Hans Sloane identified similar remains from Siberia as belonging to elephants. But what were animalsthat lived in hot climes doing in Siberia ? Itwasn't until the end of the 18th century that French zoologist Georges Cuvierconcluded that giant bones like these were from a relative of elephants thatdied out long ago - the mammoth.
So where did these mysterious giants come from? What were they like?And what drove them to extinction? Biologists have been arguing over thesequestions ever since Cuvier's time. In the past few years, however, a wealth ofnew information has emerged, thanks in part to DNA studies.
As far as fossil records go, the mammoth has one of the best, offeringan incredible insight into the evolution of this lineage. "You can tracehow the anatomy has changed from a general elephant-like animal to this veryspecialised creature that is the woolly mammoth," says Adrian Lister ofthe Natural History Museum in London
By themselves, though, bones can only tell us so much. Luckily, thefreezer-like conditions in which woolly mammoths lived and died have preservednot only bones but also flesh and hair. Sometimes entire animals have beenfound frozen, such as Lubya, a 1-month-old mammoth discovered in 2007. Not onlydo these give us a greater idea of what mammoths were like, they also preservetheir DNA blueprint. Thanks to hairs from two frozen specimens, around halfthe woolly mammoth genome has now been sequenced.
Ancient DNA is helping to fill in many of the gaps in our knowledge."In the space of just a few years, with a relatively small amount of work,we've gone from not really knowing anything at all about the movements ofmammoths to being able to say roughly when a migration happened, where theanimals came from and where they went to," says Ian Barnes, a molecularpalaeobiologist at Royal Holloway, University of London.
Mammoth DNA is also helping to settle questions about the origins ofmammoths. It has long been clear that mammoths first arose in Africa ,says Lister, as fossils of ancestral mammoths dating back as far as 5 millionyears ago have been found there. However, from anatomy alone it was not clear whetherthese ancient mammoths were more closely related to African or Asian elephants.
In 2006, three groups sequenced the woolly mammoth's mitochondrial DNA,revealing the structure of the elephant family tree. The studies show that thelineage leading to African elephants split off from the common ancestor first,around 6 million years ago. This was followed soon after by the mammothsforking away from what would become the Asian elephant (PLoSBiology, vol 8, e1000564).
These early mammoths had the spirally curved tusks characteristic oftheir kind, but otherwise probably looked much like elephants. They remainedrestricted to Africa until around 3 million years ago, when they began spreadacross Europe and Asia .
The migrants really were mammoths, as they were larger than modernelephants. Plant remains found with fossils suggest they lived in partly openhabitats, feeding on trees and bushes. There is little evidence of adaptationtowards cold in these individuals, which makes sense because the climate wasstill relatively mild. But times were changing.
Around 2.5 million years ago, an epoch of ice ages began, withtemperatures plummeting ever lower with each successive ice age. Many forestswere replaced by open grassland.
These dramatic changes led to the evolution of a new kind of mammoth,known as the steppe mammoth, with clear adaptations towards life in a colderworld and to the changing vegetation. "The steppe mammoth's teeth had moreenamel ridges to deal with a more grassy diet and a higher crown to tolerategreater wear," says Lister.
Until recently, it seemed as though this stage in the mammoth story wasa case of gradual evolution, with the first steppe mammoths appearing around750,000 years ago. But this picture was based on fossils found in Europe .
Fossils recently unearthed in China
It was around this time that some mammoths crossed a land bridgejoining Siberia to North America . Theremammoths evolved into distinctive North American forms and some eventuallyspread as far south as central America.
Meanwhile, some steppe mammoths were becoming ever more specialised forcold climates and open grassland, giving rise to the woolly mammoth, the mostfamous of its kind. Again, while fossils in Europe suggest it appearedrelatively recently, around 150,000 years ago, we now know the woolly mammothbegan evolving around 700,000 years ago in northern Siberia ,says Lister.
Its most distinctive feature was its shaggy coat, which was up to ametre long. Preserved mammoths have a wide range of hair colour, with specimenssporting blonde, red, brown and even black hair. Recent analysis of a geneknown to determine hair pigmentation, however, suggests most mammoths actuallyhad a dark-brown coat. The blonde, red and black tinges seen are the result ofdifferences in preservation, says Barnes, who was a member of the team. Thework will be published in QuaternaryScience Reviews.
Besides its long fur the woolly mammoth had a thick layer of fatbeneath its skin to insulate against the cold. It also had smaller ears and ashorter tail than its forebears to minimise heat loss. Its huge tusks wereprobably used to warn off predators and to settle disputes, but they may alsohave been used like a snowplough to expose vegetation to eat or to break upice.
The woolly mammoth's DNA is now revealing more ways in which thiscreature was adapted to life in the cold. For instance, its version of theblood protein haemoglobin was quite different to that of modern elephants, saysMichael Hofreiter of the University ofYork , UK
So woolly mammoths were built for the cold, and they thrived during aseries of ever deeper ice ages. The species spread west and east to occupy muchof the northern hemisphere, including North America ,while other mammoth species died out. Studies of mitochondrial DNA from 40woolly mammoth specimens by Barnes and colleagues show its population and rangeexpanded as the world entered the last ice age around 100,000 years ago andremained stable during the ice age itself (CurrentBiology, vol 17, p 1072). And then, as the ice age ended, it went extinct.
What happened? Some biologists think the extinction took place veryrapidly, triggered by a sudden, dramatic event around 12,000 years ago. Onesuggestion is that some kind of "megadisease" wiped out the species.Another is that a meteorite impact in North America triggered catastrophic change. And then there's the "blitzkrieghypothesis", which blames the mammoth's demise on the spread ofspear-wielding human hunters.
Hunting clearly did happen, as cave paintings and the occasionalspearhead lodged in bone testify. But there's growing evidence that woollymammoths didn't die out as suddenly as such cataclysmic visions would have usbelieve. Dating of mammoth remains by Lister and others suggest the woollymammoth's range had been in decline for several thousand years before theyfinally disappeared. And genetic studies show a loss of genetic diversity, asign of a shrinking population. This was probably a result of trees replacinggrassland as the world began to warm up again. By 12,000 years ago, woollymammoths were restricted to the steppes of Siberia ."The original, huge unbroken range of the species in its heyday shrank andbecame fragmented in ways that map onto the way the climate was changing andthe way the vegetation was changing," Lister says.
Urinating DNA
However, dating remains can only give a very rough idea of whenmammoths died out, says Eske Willerslev, an evolutionary biologist at the University of Copenhagen in Denmark
"Modern elephants urinate about 50 litres a day, which isbasically DNA all over the landscape," Willerslev says. Working on theassumption that mammoths would have done the same, Willerslev and colleaguesextracted sediment that could be accurately dated from the Alaskan tundra andlooked for mammoth DNA. Although the most recent mammoth remains from thisregion date to over 13,000 years ago, the team's findings suggest that mammothswere still living there 10,500 years ago (Proceedingsof the National Academy of Sciences, vol 106, p 22352).
That's 3500 years after the first recorded human settlement and almost2500 years after the mooted meteorite blamed for wiping out mammoths in North America . "Our data show that neither theblitzkrieg hypothesis nor the idea of an extraterrestrial impact can be thewhole story," Willersley says.
In fact, apaper due to appear in Earth-Science Reviews concludes there was noimpact after all. But climate change seems unlikely to be the whole storyeither, as mammoths had survived previous warm periods, or interglacials."It's possible that you only get extinction if you get a combination offactors coming together," says Lister.
The last stand of the mammoths took place on Wrangel Island in the Arctic Ocean . Here a population of mammoths was cut offfrom the Siberian mainland 9000 years ago as ice sheets melted and sea levelrose. However, the climate and vegetation remained suitable for them, and theysurvived here for 5000 years before dying out around 4000 years ago - aroundthe time humans arrived.
Nobody knows for sure what caused the extinction of the Wrangelmammoths. There is no direct evidence of humans killing mammoths, so it ispossible the island was simply too small to support a mammoth population.However, ancient DNA collected from the remains of some of these last mammothspaints a picture of a stable population that thrived for 5000 years and thensuddenly died out, rather than a population in terminal decline (Proceedings ofthe Royal Society B, vol 277, p 2331). So the smart money is on ourancestors having a hand in it.
The emerging consensus, then, is that as mammoths gradually became morespecialised for cold conditions and grassland, they became ever more vulnerableto climate change. Their range and population shrank dramatically as the worldwarmed, but they might well have clung on as they did in previous interglacialshad human hunters not put further pressure on them. It seems we deliveredthe final blow
What killed the megafauna?
Not long ago, giants ruled the land. The woolly mammoth was just one ofdozens of gargantuan beasts, including cave bears, giant ground sloths, Irishelk, mastodons and woolly rhinoceros. Between 40,000 years ago and the end ofthe ice age around 10,000 years ago, they vanished. Most continents lostaround 80 per cent of their large mammals forever.
Recent studies show megafauna weren't the only animals affected."All of the surviving species for which we have ancient DNA have reducedgenetic diversity at the end of the Pleistocene," says Michael Hofreiterof the University of York , UK
The cause of these extinctions has long been fiercely debated. Studiesof the mammoth suggest they were wiped out by a combination of habitat loss asthe world warmed and hunting (see main story). But that may not be truefor other animals' extinctions. "It may well be that there were differentcauses or combinations of causes for the extinction of each species," saysHofreiter.
In the case of cave bears, for example, Hofreiter and his colleagueshave found that the population began to decline around 50,000 years ago beforefinally going extinct 25,000 years ago, right at the coldest point of the lastice age. Brown bears, by contrast, remained relatively stable over the sameperiod (MolecularBiology and Evolution, vol 28, p 879). Hofreiter believes it's possiblethat humans competed directly with cave bears for suitable caves to live,pushing them out into the cold.
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